LONDON - Phase I/II data from the Astrazeneca plc COVID-19 vaccine show there were strong antibody and T cell responses to the adenoviral vectored product.

The trial run by Oxford University, showed AZD-1222 was tolerated and generated robust immune responses against the SARS-CoV-2 virus in all 1,077 healthy adult participants, aged 18-55 years. It assessed a single dose of AZD-1222 against a comparator meningococcal conjugate vaccine, MenACWY.

The results published in The Lancet confirmed a single dose of AZD-1222 resulted in a four-fold increase in antibodies to the SARS-CoV-2 virus spike protein in 95% of participants one month after injection.

T cell responses targeting the SARS-CoV-2 spike protein were markedly increased, peaking 14 days after vaccination, compared to placebo. The level had declined slightly by day 56 of the trial. The T cell response did not increase in 10 subjects who received a second dose of the vaccine, which the researchers at Oxford University say is consistent with other vaccines of this kind.

Antibody responses peaked by day 28 and remained high until day 56. By 28 days after vaccination, neutralizing antibody responses against SARS-CoV-2 were detected in 32 of 35 participants using one neutralizing assay, and in 35 of 35 participants when measured by another assay.

Sarah Gilbert, co-author of the paper said there is “still much work to be done” before the vaccine is shown to be effective, “But these early results hold promise,” she said.

As well as continuing to test the vaccine in phase III trials, there is a need to know the correlates of protection that will make for an effective product. “If our vaccine is effective, it is a promising option as [it] can be manufactured at large scale,” Gilbert said.

Andrew Pollard, chief investigator said, “The immune responses observed following vaccination are in line with what we expect will be associated with protection against the SARS-CoV-2 virus, although we must continue with our rigorous clinical trial programme to confirm this. We saw the strongest immune response in participants who received two doses of the vaccine, indicating that this might be a good strategy for vaccination.”

The authors note some limitations, including that more research is needed to confirm their findings in different groups, including older age groups, those with other health conditions, and in ethnically and geographically diverse populations.

These volunteers are these groups are being recruited in the ongoing phase II and III trials in the UK, Brazil and South Africa. In the current trial, which took place in the U.K., 91%, or 979 of 1,077 participants were white and the average age of participants was 35 years.

A small number of participants had detectable neutralizing antibodies and T cell responses against SARS-CoV-2 spike protein before vaccination, likely due to past asymptomatic infection, as potential participants with recent COVID-19-like symptoms, or with a positive PCR test for SARS-CoV-2 were excluded from the study.

The participants will be followed-up for at least one year to continue to study the vaccine’s safety and the immune response it provokes.

“We are encouraged by the phase I/II interim data showing AZD-1222 was capable of generating a rapid antibody and T-cell response against SARS-CoV-2. While there is more work to be done, today’s data increases our confidence that the vaccine will work and allows us to continue our plans to manufacture the vaccine at scale for broad and equitable access around the world,” said Mene Pangalos, executive vice president of biopharmaceuticals R&D at Astrazeneca.